1. Field of the Invention
This invention relates to aqueous nonyellowing coating compositions usable, for example, for top coat lacquers. They may be used as water dilutable uni- or metallic base coats and covered in the usual manner by spraying with a clear lacquer which is dilutable with the water or dissolved in organic solvents. The binders may be used as industrial lacquers, especially, for example, in the motor car industry. They result in coatings which in addition to providing a good optical effect and having excellent mechanical properties can be applied trouble free by electrostatic spraying, e.g. with high rotation spray apparatus. They are also suitable for repair purposes as they provide excellent properties even when hardened only at low temperatures such as 80.degree. C.
2. Description of the Related Art
Conventional water dilutable polyacrylate resins have a comparatively low molecular weight and contain numerous hydrophilic groups. The addition of neutralizing agents results in polycarboxylic acid salts which are soluble in water or dilutable with water. These resins must be cross-linked with cross-linking agents such as melamine resins and/or blocked polyisocyanates at elevated temperatures. These reactions proceed so slowly at temperatures below 100.degree. C. that the films show loss of adherence to their base or blistering on the surface when exposed to moisture.
Resins having a relatively high molecular weight are prepared by emulsion polymerization in an aqueous medium. The addition of surface active agents or emulsifiers gives rise to problems in the water resistance of the films. Electrostatic spraying entails difficulties owing to their tendency to coagulate.
Water dilutable binder systems containing polymer microparticles which are insoluble in nonaqueous solvents and water are described in DE-OS No. 28 60 661 (U.S. Pat. No. 4,209,435). They are prepared with the aid of polymeric steric dispersion stabilizers by polymerization in nonaqueous solvents and subsequently converted into the aqueous medium. This conversion process is highly complicated and susceptible to trouble because the action of the dispersion stabilizer is impaired by variations in the manufacturing process. In EP-A-38l27, these binders are used for the preparation of metallic base coats, the aluminum platelets or pigments being introduced into the lacquer by means of a melamine resin.
Primers for metal coils based on acrylate resin/polyurethane dispersions are described in DE-PS 27 36 542 (U.S. Pat. No. 4,103,050). No special acrylate resins adjusted to the requirements of metal base coats are mentioned. DE-OS 23 63 307 describes the emulsion polymerization of vinyl monomers in polyurethane dispersions. These vinyl monomers may be acrylate esters; the use of vinyl monomers containing free carboxyl groups is not mentioned, nor is there any mention of special requirements for metallic base coats. The high molecular weight products obtained by emulsion polymerization are used as modified polyurethane rubbers.
In DE-OS No. 32 10 051, water dilutable polyurethane dispersions are used for the preparation of metallic base coats. The films obtained from them are generally too soft and must therefore be cross-linked with melamine resins. It is therefore difficult to obtain suitable water resistant film properties at the low temperatures which are necessary for repairs. Electrostatic spraying also gives rise to difficulties.
The problem therefore existed of providing a binder system which would to a large extent be physically drying and would not have the disadvantages mentioned above.